1. Field of the Invention
The present invention relates to a method of manufacturing a steel product such as a thick steel plate, steel strip, shape steel, steel bar and the like io used in the fields of construction, ocean structures, pipes, ship building, reservoirs, civil engineering, construction machinery and the like, and, in particular, a thick steel product of high strength and high toughness having excellent weldability and minimal variation of structure and physical properties.
2. Description of the Related Art
A thick steel product such as thick steel plate has been used in various fields as described above and the characteristics thereof such as increased strength and toughness have been improved. In particular, recently, it is required that these characteristics are uniform in a thickness direction of the product, and less variable among a plurality of steel products.
One reason for that requirement is illustrated by the fact that, as buildings are made increasingly tall, they are designed so that vibration energy resulting from a large earthquake is absorbed by the controlled deformation of a building to prevent its chaotic collapse, as described in "Iron and Steel, 1988, No. 6" ("Testu to Hagane Dai 74 Nen (1988), Dai 6 Gou"), page 11-page 21. More specifically, when an earthquake occurs, the framework of the building is partially collapsed in a predetermined shape so that the total or chaotic collapse of the building is prevented by the plasticization of the framework. However, since this idea is based on the premise that when an earthquake occurs, the framework of a building exhibits a behavior intended by a designer, the designer must know precisely the yield strength ratio of the steel products used for the columns, beams and the like of the building. Therefore, it is indispensable that steel products such as steel plates, H-sections and the like used for the columns, beams and the like are uniform, and variation in the strength of the steel products is a serious problem.
Since it is necessary that steel products used for building and ship building have high tensile strength and high toughness, it is conventional to manufacture this type of steel product by a thermo-mechanical control process (hereinafter referred to as TMCP method). However, when thick steel products are made by the TMCP method, the structure of them is varied because the cooling rate in a cooling process executed after rolling is different along the thickness direction of a given product, or among several such products. This problem occurs because the cooling rate is large in the vicinity of the surface of the steel products when they are cooled, whereas the cooling rate is small at the center of the steel products, in thickness direction thereof. As a result, the material of the thus obtained steel products varies along the thickness direction of a given piece, and/or among a plurality of pieces. The variation of the material appears between the webs and between the flanges of an H-section due to the irregular cooling therebetween or among respective lots; additionally, it appears as a particular problem along the thickness direction of a thick steel plate.
To cope with the above problem, Japanese Unexamined Patent Publication No. 63-179020 discloses a method of reducing the difference of hardness of the cross section of a steel plate in a thickness direction by controlling components, a rolling reduction ratio, a cooling rate and a cooling finishing temperature. However, when a thick steel plate, in particular, a very thick steel plate having a thickness exceeding 50 mm is made, since a cooling rate inevitably varies along the thickness direction thereof, it is difficult to suppress the difference of hardness of the cross section in the plate thickness direction.
Japanese Unexamined Patent Publication No. 61-67717 discloses a method of greatly reducing the difference of strength in a plate thickness direction by greatly reducing a C content. As shown in FIG. 3 of the publication, however, the method cannot correct the variation of strength caused by the change of a cooling rate which inevitably arises particularly in a thick steel plate.
Japanese Unexamined Patent Publication No. 58-77528 describes that stable distribution of hardness is obtained by the complex addition of Nb and B. However, since the cooling rate must be controlled to the range of 15-40.degree. C./sec to form bainite, and it is difficult to strictly control the cooling rate at the center of a plate in the thickness direction thereof, there is a problem that a uniform microstructure cannot be obtained in the thickness direction of the plate, strength is variable, and ductility and toughness are deteriorated due to the formation of island-shaped martensite.
Furthermore, it is important that the steel product used for the above applications have high toughness and a tensile strength greater than 570 MPa. For this purpose, a method of obtaining a fine tempered martensitic structure by a process of reheating, quenching and tempering has been mainly used. However, this method has a problem in that high cost is associated with the reheating, quenching and tempering process and further since a weld cracking parameter (hereinafter referred to as P.sub.cm), which is the index of weldability, increases due to an increased quenching property, and weldability is thereby deteriorated.
On the other hand, Japanese Unexamined Patent Publication No. 62-158817 discloses a method of obtaining a thick steel plate having high strength at a relatively low P.sub.cm by executing a tempering process after rapid cooling while using the precipitation of Nb and Ti. In this method, however, there is a fear that distortion is caused by irregular cooling in addition to the high cost of a quenching and tempering process.
Likewise, although Japanese Unexamined Patent Publication No. 55-100960 discloses steel whose weldability is enhanced by regulating P.sub.cm and limiting the amounts of C, N and S, it is difficult to prevent the significant variation in strength along the thickness direction thereof.
Further, Japanese Unexamined Patent Publication No. 54-132421 discloses making high tension bainite steel by hot rolling executing at a finishing temperature of 800.degree. C. or less to obtain toughness, and greatly reducing a C content to use the steel as pipeline raw material. However, this method has a problem that since the hot rolling is finished in a low temperature region, when a plate must be slit lengthwise, not only distortion and warping are liable to be caused by the slitting but also variation arises between the strength in a rolling direction (L direction) and the strength in the direction perpendicular to the L direction (C direction) by the rolling executed in the low temperature region.
An object of the present invention is to provide a method of manufacturing a steel product free from the above problems, that is, a steel product which is not restricted by the cooling rate after rolling, has minimal variation of microstructure along its thickness direction and among plural products, is excellent in weldability and has high toughness of 570 MPa or more in terms of tensile strength.